Adsorbent units are utilized to adsorb one or more components that are contained in a feed stream to produce a product stream. The adsorption is carried out in adsorbent beds that are operated in and out of phase cycle such that while one bed is on-line and adsorbing component or components of the feed stream, another off-line bed is being regenerated. There are a variety of cycles that are employed in such adsorbent units, for example, pressure swing adsorption, temperature swing adsorption and vacuum pressure swing adsorption.
In a pressure swing adsorption cycle, the off-line bed being regenerated is subjected to a blow down phase of the regeneration in which the bed is depressurized. Thereafter, the bed is purged with a stream that has a sufficiently low concentration of the component or components to be adsorbed that the adsorbed component or components will desorb and be carried off the bed in the purge stream. This is known as the purge phase of the cycle. After the purge phase of the cycle is complete, the bed is brought back up to pressure with either part of the feed stream or part of the product stream. This phase is known as repressurization. Other cycles such as temperature swing adsorption and vacuum pressure swing adsorption require repressurization of the off-line bed to bring it back to operational pressure.
During repressurization, either part of the product stream or part of the feed stream is introduced into the off-line bed. In this regard, a repressurization valve is opened sending part of the compressed feed stream or the product stream into the off-line bed. Such a valve is a flow control valve that is controlled by a flow controller so that the flow rate of the product stream is maintained within a target range. As can be appreciated, the flow rate of the product stream will nevertheless vary.
This fluctuation can have an affect on downstream process equipment in which the product stream is utilized. For example, pressure and temperature swing adsorption systems are used in connection with cryogenic air separation plants. Since cryogenic air separation plants operate at a low temperature, it is necessary to remove higher boiling contaminants within the air, for example, carbon dioxide, water vapor and hydrocarbons. Such contaminants could freeze or in case of hydrocarbons, could reach dangerous concentrations within oxygen products. In order to counteract this problem, known adsorbents contained within adsorbent beds are utilized to purify a feed air stream from such contaminants. However, the fluctuation of flow rate in the product stream, which in case of a cryogenic air separation plant is the compressed and purified air stream, will affect product purities and product recoveries that have been found to fluctuate in accordance with the flow fluctuation in the compressed and purified air stream. In order to minimize such fluctuations, control schemes have been devised in which the fluctuation of the product stream flow rate is minimized by increasing the flow rate of the compressed feed stream fed from a compressor to the adsorption unit to counteract either the feed stream or the product stream being drawn off for repressurization purposes.
In one type of control scheme employed in the prior art, the compressed feed stream flow rate is increased and then decreased at fixed rates of increase and decrease. The increase and decrease is brought about by increasing and decreasing the inlet guide vane opening within the compressor feeding compressed air to the adsorption unit. The operating principle here is that as the repressurization valve is initially opened, there exists a high pressure difference between the off-line and the on-line bed. As the pressures approach equalization less driving force is available. At a pre-defined off-line to on-line triggering pressure ratio, that can be as high as 80 percent, the off-line adsorbent bed is nearly repressurized and therefore, the flow rate can be decreased rapidly. This, however, has been found to lead to a repressurization performance that varies due to inherent variability of ambient air pressure, temperature conditions, contaminant concentration in the air, bed performance, valve wear and air compressor performance variation due to cooling water temperature change.
In a specific control scheme designed to remove the effects of external factors, such as those outlined above, the bed pressures between an off-line bed and an online bed are measured and a pressure ratio is computed. This pressure ratio is continually compared to a preset triggering pressure ratio. The inlet guide vanes of the main air compressor are then manipulated so that flow rate of the feed stream increases until the triggering pressure ratio is reached and then decreases back to the nominal flow rate. The control program governing the operation of a controller used in controlling the inlet guide vanes utilizes an increase rate function that is a function of the bed pressure ratio and the triggering pressure ratio to eliminate the effect of the external factors. Additionally such increase rate function is designed so that the rate of increase of the opening of the inlet guide vanes will decrease as the triggering pressure ratio is approached. The purposes of such operation is to eliminate inertial effects of the compressor wheel that will inhibit the increasing flow rate from being decreased after the triggering pressure ratio is reached and also to minimally vary the flow rate of the compressed feed stream as required to repressurize the off-line bed. After the increase, the opening of the inlet guide vanes is decreased in accordance with a fixed rate until the nominal flow rate is reached. The problem remaining in such type of control is that the decrease is still effected by external factors and inertial effects can produce an overshoot as the flow rate of the compressed air stream is returned to the nominal flow rate.
An additional problem with the control scheme discussed above is that the response of product flow rate is not equal with respect to each of the beds and therefore, depending upon the bed, there will be more or less disturbance in the product flow rate. A related issue concerns the need to reduce the repressurization time. A reduction of the time spent in repressurizing an adsorbent bed will help prolong the purge phase time and will in turn extend the adsorption time because a longer purge time will result in a cleaner adsorbent bed. Extending the adsorption time will thereby reduce the frequency of bed switches and resulting valve wear and disturbances in product stream flow that are propagated to downstream equipment utilizing the product of the adsorption. Put another way, decreasing the repressurization time will allow for an increase of the bed cycle time. Increasing the bed cycle time is important in case of an air separation plant to eliminate the number of times adsorbent beds are switched from on-line and off-line status. However, reductions in the repressurization time usually lead to amplified air flow and/or pressure disturbances reaching downstream equipment that may be particularly sensitive to the same, for example, air separation units. The reason for this is the large number of separate items of equipment involved, for example, two or more adsorbent beds, valves and air compressor and etc. Moreover, the repressurization time cannot be reduced to a value that will produce fluidization of the adsorbent within the adsorbent beds.
It is to be noted that adsorbent bed cycle time is routinely adjusted in the prior art for maintaining the purity of the product stream. For instance, if the purity of the product stream decreases, the bed cycle time can be decreased and vice-versa. However, since in prior art control schemes, such as discussed above, repressurization time is not controlled, the adsorbent beds will not be equally regenerated because the time spent in purging the adsorbent beds will vary. As a result, the ability to extend bed cycle times will be limited by the adsorbent bed that has been least regenerated. As can be appreciated, the least regenerated adsorbent bed will not be able to remain in an on-line mode of operation adsorbing the impurities within the feed as long as a bed that has been more regenerated because the least regenerated adsorbent bed contains more impurities after regeneration.
As will be discussed, among other advantages, the present invention allows control to be exerted over the repressurization of adsorbent beds such that product flow rate disturbances are minimized and adsorbent bed repressurization times are driven towards a target to minimize repressurization times and to allow the operation of the adsorption unit to be optimized.